December 23, 2010

Neandertals not cold-adapted (?)

There was a paper in 2008 which explained the wide nasal aperture in Neandertals in the context of their cold adaptation. That paper suggested that a combination of broad and long nose was the Neandertal solution to the cold.

Here comes a paper which argues against cold adaptation as the explanation for Neandertal facial morphology. On purely evolutionary grounds it's difficult to see why Neandertals would not be facially cold-adapted as they did live in extreme cold conditions. But, I'd have to look at the case the authors are making before voicing my opinion.

Journal of Human Evolution doi:10.1016/j.jhevol.2010.10.00

The Neanderthal face is not cold adapted

Todd C. Rae et al.

Many morphological features of the Pleistocene fossil hominin Homo neanderthalensis, including the reputed large size of its paranasal sinuses, have been interpreted as adaptations to extreme cold, as some Neanderthals lived in Europe during glacial periods. This interpretation of sinus evolution rests on two assumptions: that increased craniofacial pneumatization is an adaptation to lower ambient temperatures, and that Neanderthals have relatively large sinuses. Analysis of humans, other primates, and rodents, however, suggests that the first assumption is suspect; at least the maxillary sinus undergoes a significant reduction in volume in extreme cold, in both wild and laboratory conditions. The second assumption, that Neanderthal sinuses are large, extensive, or even ‘hyperpneumatized,’ has held sway since the first specimen was described and has been interpreted as the causal explanation for some of the distinctive aspects of Neanderthal facial form, but has never been evaluated with respect to scaling. To test the latter assumption, previously published measurements from two-dimensional (2D) X-rays and new three-dimensional (3D) data from computed tomography (CT) of Neanderthals and temperate-climate European Homo sapiens are regressed against cranial size to determine the relative size of their sinuses. The 2D data reveal a degree of craniofacial pneumatization in Neanderthals that is both commensurate with the size of the cranium and comparable in scale with that seen in temperate climate H. sapiens. The 3D analysis of CT data from a smaller sample supports this conclusion. These results suggest that the distinctive Neanderthal face cannot be interpreted as a direct result of increased pneumatization, nor is it likely to be an adaptation to resist cold stress; an alternative explanation is thus required.

7 comments:

The Northern temperature bound of Neanderthals does seem to have been milder than the Northern temperature bound of successor modern human hunter-gatherers.

But, the Neanderthals also had to lean much more heavily on biology than modern humans because they had far less technology to deal with it. They didn't have hats or shoes. They probably did have woolen or cotton clothing. They didn't have pack animals, so they couldn't carry much with them of any kind as they moved. They didn't have any significant man made structures, so when they were away from caves they were pretty much out in the open. Yes, they had fires and each other, but a "temperate" climate can be pretty darned cold without adequate clothing or shelter, and relying on body fat is troublesome when your diet is irregular because your hunting results are irregular.

In short, the same region might be effectively colder for Neanderthals than it was for modern humans, because Neanderthals would be routinely exposed to lower temperature than modern humans who would have had better clothing and shelter to protect them at the coldest moments.

I'm also a little skeptical of efforts to scale up or scale down Neanderthal features because, due to cube-square issues, scale matters. A scale model of a spider that was five feet long would collapse under its own weight. The properties of a nasal cavity at modern human size might be different at Neanderthal size.

sinus capacity may have been for food and water sensing. when i was long distance walking i could smell a very tiny stream about 300 yards away. i had no reason to believe it was there and just walked down the hill and found it. also recently i was walking in the woods and i smelled something that seemed like it would be edible, i couldn't find it but i smelled something that seemed like i could eat it, some kind of plant. the hungrier and thirstier you get the better your sense of smell becomes.

Maybe the big conks of Neanderthals were not meant for cold adaptation per se but just for the humidification of otherwise dry air.

Look around the world at peoples with big conks: New Guinea highlanders, certain North American Indian tribes, Arabs from desert regions, some Southern Europeans. It is interesting that Northern Europeans, especially the eastern ones, by and large have small but broad noses. Less to be frost bitten? It is interesting that many humans that have lived in cold environments don't have huge conks.

My hypothesis is that the big conks of Neanderthals was used to humidify air which in that frozen environment would have been extremely dry. Warming the air was of secondary importance.

They certainly used leather/fur to keep their feet and head warm - even if these were likely not as detailed as what AMHs put together. There was a paper a while ago that looked at the energy balance of Neanderthals and came to the conclusion that they must have had (i) pretty well-insulating closing and (ii) living structures (tents and/or huts). There is a French site associated with Neanderthals that shows construction of wooden palisades and living structures, and a German site (Bilzingsleben) that has stone rings interpreted as remnants of tents, as well as a large circle laid out with a stone "tile" floor from Heidelbergensis (~400,000 years ago - climate slightly warmer than Germany, now).

I agree though that AMH's technology was far superior, and they managed to make a living in some extremely cold environments (without caves).

Finally, no one argues that Neanderthals were not cold-adapted - they clearly were (short lower extremities, short neck, rounder upper body). Yet, their nose may not have been.

This seems a little misleading, at least from the abstract, inasfar as it isn't really saying that the Neanderthal face is not cold selected, merely that their pneumatisation neither is particularly outstanding, nor cold selected.

However, it is still fairly plausible that their nasal cavities are cold selected.

Perhaps a better title would be "Neanderthal pneumatisation is not a sign of cold adaptation"?

Interestingly inasfar as I can tell from a quick google, the effect of temperature on the size of the maxilliary sinus they describe (as the sole effect correlation between temperature and the sinus system) seems explained in macaques by expansion of the nasal cavity in cooler climates forcing a smaller maxilliary sinus - http://www.ncbi.nlm.nih.gov/pubmed/12682923, actually by Rae, the author of this paper.

If there is no relationship between pneumatisation and sinus size except in cases in which nasal cavities compete with maxilliary sinuses, and Neanderthal nasal cavities managed to increase in size without exerting competing pressures on their maxilliary sinues, then it seems quite odd to use pneumatisation as evidence that the Neanderthal face was not cold adapted.

I am also not too sure that European modern humans are necessarily the best or a even a good sole comparison group to determine whether pneumatisation is proportional in volume to skull size. Other modern homo sapiens of more diverse origins and fossils from erectine groups would seem to allow a richer and better comparison group (although I guess that's a function of a limited pool of subjects and limited resources).

The Middle-Eastern and other West-Asian neanderthal subtypes did not differ much from their European counterparts. I would assume this would indicate that climactic conditions weren't a precursor to changes in facial morphology. For all we know, Homo Heidelbergensis, the disputed common ancestor between sapiens and neanderthal retained archaic facial characteristics that were precursors to both species. Since heidelbergensis fossils have been found in North Africa as well, it goes to show that nasal structure, sinus volume, are not exactly indicative of cold weather adaptation. We're obviously discounting altitude with latitude. It's possible that it was a cold weather adaptation, but not indicative of latitude, but an evolution in mountainous regions that led to their distinctive facial morphology.

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